With the development of Global Navigation Satellite System (GNSS), the requirement of navigation services is increasing. The number of signals transmitted on the same frequency band by the navigation satellite systems is growing, which aggravates the crowding of the already limited frequency band available for the satellite navigation. With growing in the number of signals broadcast in the same frequency band by a navigation system, the complexity of the satellite payload keeps increasing.
It is desired to multiplex signals on two different frequencies to certain specific requirements in the system construction and application for, e.g., the smooth transition of the adjustment in the central frequency of signal during the system update and upgrade, or transmission of multiple groups of service information with contents complimentary to each other onto two very adjacent frequencies, etc. Moreover, under the condition that the transmitting power from satellite is limited, in order to keep enough receiving power at the receiver end, it is desired that the high power transmitter on the satellite have as high power efficiency as possible. Thus, it is required that the High Power Amplifier (HPA) on the satellite keep working in the non-linear saturated region. However, when the HPA works near the saturated point, if the input signal does not have a constant envelope, the output components will be subject to such distortions as amplitude modulation, amplitude-phase conversion, and so on, which will cause the amplitude and phase distortion in the transmitting signal and seriously affect the performance of the receiving end. Therefore, it is required to ensure that the combined signal has constant envelope.
As a typical instance, AltBOC (U.S. patent application US2006/0038716A1), a constant envelope modulation technique, is applied in the signal on E5 band of European Galileo navigation system. In AltBOC, two sets of BPSK-R(10) signals respectively modulated on two separated carrier frequencies with 30.69 MHz away from each other (E5a: 1176.45 MHz, E5b: 1207.14 MHz) are combined into a constant enveloped 8PSK signal with central frequency at 1191.795 MHz. By such technique, advantageously, the number of signal transmitters carried as the payload of satellite is saved, and a wideband multiplexed signal is constructed, such that the receiver supports not only the narrowband receiving strategy by which the signal components on E5a and E5b are separately received and processed, but also the wideband receiving strategy by which the integral multiplexed signal in its full band is received for a better ranging performance. However, in AltBOC, the four signal components to be multiplexed must have equal power, which restricts the flexibility in application of the technique. As known in the GNSS, since ranging is the primary purpose of GNSS signal, it tends to allocate more power on the pilot channel than the data channel in the GNSS signal structure design, in order to promote the accuracy and robustness of pseudorange measurements and carrier phase tracking. Moreover, the adoption of different spreading code chip waveforms by signal components (such as BPSK-R, sine-phased BOC, cosine-phased BOC, TMBOC, QMBOC, etc.) results in the different performance in acquisition, tracking and data demodulation at the receiver end. Therefore, it is required to provide a dual-frequency constant envelope multiplex technique for GNSS signals which is more flexible than AltBOC, in particular, such that the four components can be different in power allocation and the spreading code chip waveform of different signal components can be flexibly selected.
PCT international patent application no. PCT/CN2013/000675, with the title of “Satellite Navigation Signal and Generation Method, Generation Device, Receiving Method and Receiving Device Therefor”, discloses a method for generating a multiplexed signal with constant envelope based on the values and power ratio of four signal components to be multiplexed. According to this method, the in-phase baseband component and quadrature-phase baseband component of the multiplexed signal satisfying the requirement for constant envelope can be calculated. However, the calculation of the in-phase baseband component and quadrature-phase baseband component of the multiplexed signal in a satellite navigation signal generating device will result in the increase of the complexity in implementing the device.